by Polyethylene is one of the most common types of plastic in the world.
While it’s easy to make, it’s devilishly difficult to break again, making reuse and recycling difficult.
But a team of US chemists has found a new way to turn polyethylene back into parts – a breakthrough that could one day make it fully circular.
“To the extent that they are recycled, many polyethylene plastics are turned into low-quality materials. You can’t take a plastic bag and then make another plastic bag with the same properties,” says Professor John Hartwig, an organometallic chemist at the University of California, Berkeley, in the US.
Hartwig is senior author on a paper at Science, describing how the long molecular chain (polymer) of polyethylene can be broken down into propylene, also called propene: a useful raw material for a variety of different things.
“If you can take that polymer bag back to its monomers, break it into small pieces and repolymerize it, then instead of pulling more carbon out of the ground, you’re using it as a carbon source to make other things – for example, polypropylene says Hartwig.
“We would use less shale gas for this purpose or for other uses of propene and to fill the so-called propylene gap.”
Read more about polythene recycling: Plastic-eating caterpillar could destroy landfill
Polyethylene is made of carbon atoms linked together in a long chain, with hydrogen atoms branching off to the side.
The bond between the carbon atoms – the carbon-carbon bond – is difficult to break and even broken in a systematic way.
The researchers’ innovation was to use a few different metals to catalyze two different reactions.
The first relies on a catalyst made of iridium, or platinum and zinc, to modify the resistant carbon-carbon bond.
“We take a saturated hydrocarbon—all carbon-carbon single bonds—and remove some hydrogen molecules from the polymer to create carbon-carbon double bonds, which are more reactive than carbon-carbon single bonds,” Hartwig says.
“A few people had looked at this process, but no one had succeeded in a real polymer.”
The researchers then found that a palladium-based catalyst could pounce on that bond and use it to gradually break down the polymer with a substance called ethylene.
“Once we have a long chain with a carbon-carbon double bond at the end, our catalyst takes that carbon-carbon double bond and isomerizes it, with one carbon in it,” says Hartwig.
“Ethylene reacts with this initial isomer product to make propylene and an almost identical, slightly shorter, terminal double bond polymer.
“And then he does the same thing over and over again. He takes a step inside, cuts. goes in, breaks down. it goes in and cuts until the entire polymer is cut into three-carbon pieces. On one end of the chain, it just chews up the chain and spits out propylene until there’s no chain left.”
More on plastic pollution: Experts use model to offer radical solution to plastic waste: don’t reduce – just stop
They managed to convert 80% of their polyethylene into propylene: small molecules with three carbon atoms each.
There is still a lot of work to be done before the process can be industrialized.
Right now, for example, both catalysts must be in liquid form—researchers hope to find solid catalysts instead, because they’re easier to reuse.
Hartwig says the technique is “far from commercialization.”
“But it’s easy to see how this new process will turn the largest amount of plastic waste into a huge chemical feedstock — with much further development, of course.”
